This invention relates to a system for controlling an ammonia injection rate for an apparatus for removing nitrogen oxides (NO.sub.x) from a combustion flue gas discharged from a waste heat recovery boiler in a combined power plant.
Large capacity thermal power plants have been under construction to meet increasing power demands, and it is required that their boilers are operated under various pressures to obtain a high power generation efficiency even under a partial load, because the thermal power generation tends to shift from a base load operation to a load adjustment owing to the increasing atomic power generation and also to the increasing difference between the maximum load and the minimum load, which are characteristic of the recent power demands. That is, in the thermal power plants, the boilers are operated not always under a full load, but under variable load of 75%, 50%, or 25% full load in the daytime, or the boiler operation is discontinued in the night, for example, according to the so called Daily Start-Stop operation schedule, which will be hereinafter referred to merely as "DSS operation schedule", whereby such intermediate loads are borne to improve the power generation efficiency.
As part of improvements in the highly efficient power generation, combined power plants have been recently regarded as important. In the combined power plants, power generation is carried out by a gas turbine, and the heat possessed by the flue gas discharged from the gas turbine is recovered in a waste heat recovery boiler, while the steam generated in the waste heat recovery boiler drives a steam turbine also to conduct the power generation.
The combined power plant has a high power generation efficiency owing to power generation by both gas turbine and steam turbine, and also has a good load response. Thus, the combined power plant has a good load follow up capable of fully meeting a drastic increase in the power demand as an advantage and is particularly effective for the DSS operation schedule.
However, in the combined power plant, so called clean fuel such as LNG, kerosene, etc. is used, and thus SO.sub.x or dusts is less evolved, but the combustion in the gas turbine requires much combustion air, and the amount of NO.sub.x is increased in the flue gas due to the high temperature combustion. Thus, a waste heat recovery boiler provided with an apparatus for removing nitrogen oxides has been used (U.S. Pat. No. 4,466,241).
In FIG. 2, a combined power plant is schematically shown, where numeral 1 is a gas turbine, 2 a flue gas passage for a flue gas G from the gas turbine 1, 3 a superheater, 4 a primary evaporator, 5 an apparatus for removing nitrogen oxides, 6 a secondary evaporator, and 7 an economizer. The superheater 3, the primary and secondary evaporators 4 and 6, the apparatus for removing nitrogen oxides 5 and the economizer 7 are provided in the flue gas passage 2. Numeral 8 is a steam-generating drum, 9 a steam turbine driven by the steam generated in the drum 8, 10 a condenser for condensing the steam to water, and 11 a condensate pump for feeding the water from the condenser 10 to the drum 8.
The water in the condenser 10 is led to the economizer 7 by the condensate pump 11 as feedwater W, preheated with the flue gas G in the economizer 7, and then fed to the drum 8. The water in the drum 8 goes downwards through a descending pipe 13 and led to the evaporators 4 and 6 through lines 14a and 14b, respectively, and returned to the drum 8 through lines 15a and 15b, respectively. During the cyclic flow of the water, steam generated in the evaporators 4 and 6 by heating with the flue gas G is led to the superheater 3 from the drum 8 through a saturated steam pipe 16, and superheated by the flue gas G and fed to the steam turbine 9 through a main steam pipe 17. Numeral 18 is a turbine bypass pipe branched from the main steam pipe 17 to lead the steam directly to the condenser 10 while bypassing the steam turbine 9. Numeral 19 is a steam turbine control valve for controlling the feed rate of steam to the steam turbine 9, 20 a turbine bypass valve for controlling a bypass rate of steam by the feed rate of steam to the steam turbine 9, and 21 a damper in the flue gas passage 2.
In the foregoing, the combined power plant has been outlined. Generally the waste heat recovery boiler is provided with an apparatus for removing nitrogen oxides 5 therein to recover the heat from the flue gas and remove nitrogen oxides simultaneously. In order to attain an effective nitrogen oxides-removing action of the catalyst filled in the apparatus for removing nitrogen oxides, it is necessary to keep the catalyst within a specific temperature range. Outside the specific temper-ature range, the nitrogen oxide-removing effect of the catalyst is reduced.
FIGS. 3 (a), 3(b), 3(c) and 3(d) show characteristic diagrams between the gas turbine load on the axis of abscissa and leaked NH.sub.3, inlet NO.sub.x, flue gas temperature or flue gas flow rate on the axis of ordinate, where curve A is plotted for changes in the flue gas flow rate, curve B for changes in the flue gas temperature, curve C for inlet NO.sub.x (NO.sub.xi), curve D for NO.sub.x in terms of 6% O.sub.2, curve E for changes in leaked NH.sub.3, and straight line F for the control value of leaked NH.sub.3. In FIG. 3 (c), T.sub.1 and T.sub.2 show a temperature range for effective nitrogen oxides-removing action of the catalyst, and in FIG. 3 (d) point H shows a design point.
The temperature range between T.sub.1 and T.sub.2 is generally from 180.degree. to 400.degree. C., but a higher catalyst activity is obtained substantially around 350.degree. C. However, as already explained referring to FIG. 2, heat transfer pipes of superheater 3 for superheating steam and evaporator 4 for evaporating feedwater are provided on the upsteam side of the apparatus for removing nitrogen oxides 5, and thus the flue gas is deprived of the heat by these heat transfer pipes of superheater 3 and evaporator 4 and the inlet temperature of the apparatus for removing nitrogen oxides 5 is considerably lowered at a low load operation or at a low flue gas rate. Thus, the design point H of the apparatus for removing nitrogen oxides 5 will be at a lower flue gas temperature with a lower catalyst activity, as shown in FIG. 3 (c), and the catalyst amount and NH.sub.3 injection molar ratio will be set to meet these conditions.
However, when a gas turbine load is increased as shown by curve A in FIG. 3 (d), the inlet NO.sub.x will be increased as shown by curve C in FIG. 3 (b). Thus, when the constant mole ratio control based on the maximum mole ratio set at the design point H is carried out, as in a conventional manner, the leaked NH.sub.3 (S-NH.sub.3) will be increased with increasing load as shown by curve E in FIG. 3 (a). Curve E will exceed the control value F.
The leaked NH.sub.3 can be given by the following equation (1): EQU S--NH.sub.3 =(M--.eta.).times.NO.sub.xi ( 1)
wherein
S--NH.sub.3 : leaked NH.sub.3 (ppm) PA1 M: set mole ratio (-) PA1 .eta.: nitrogen oxide removal efficiency (-) Flue gas temperature increase with increasing load, and the nitrogen oxide removal efficiency is somewhat improved (no considerable increase). PA1 NO.sub.xi : inlet NO.sub.x (ppm) PA1 NO.sub.xi : inlet NO.sub.x (ppm) PA1 NO.sub.xo : outlet NO.sub.x (ppm) PA1 S-NH.sub.3 : leaked NH.sub.3 (ppm) PA1 .eta.: nitrogen oxide removal efficiency (-)
On the other hand, when the conventional constant outlet NO.sub.x control is carried out, the amount of NH.sub.3 to be injected can be reduced, and most economical operation can be made, but when there are considerable changes in load as in DSS operation schedule in a combined power plant, there is a less allowance between the outlet NO.sub.x and the control value, leading to poor follow up and such a disadvantage that the outlet NO.sub.x concentration exceeds the control value.
Generally, NH.sub.3 /NO.sub.x mole ratio for NH.sub.3 to be injected to remove nitrogen oxides can be represented by the following equation (2), and thus must be set to meet the inlet NO.sub.xi : ##EQU1## wherein: M': NH.sub.3 /NO.sub.x mole ratio (-)
However, in the case of an apparatus for removing nitrogen oxides for a gas turbine, changes in load are vigorous, and when the mole ratio is set by equation (2), there is a less allowance for the upper limit to the outlet NO.sub.x, and thus there is a high possibility that the outlet NO.sub.x exceeds the control value.